This invention relates to fiber optic couplers for providing signal connections between plural optical fibers. More specifically, it relates to such couplers in which multiple optical fibers are fusion bonded in order to provide a distribution of signals between the fibers. An example of such a coupler would be a bidirectional access coupler for single strand fibers.
There are currently two fiber optic configurations for couplers in use. One is a serial distribution system that employs T-access couplers, and the other is a parallel system employing a star coupler. Both 1 dB insertion loss cable connectors and 7 dB insertion loss star couplers suitable for bundle systems have been reported in the open literature. See, for example., an article by Frank L. Thiel, Roy E. Love, and Rex L. Smith in the "Journal of Applied Optics", Volume 13, page 240 (1974); and an article by M. C. Hudson and F. L. Thiel, "Journal of Applied Optics", Volume 13, page 2540 (1974).
It can be shown that generally the signal level advantage of the star format over the serial format increases as the number of terminals in the system increases. The advantage is more pronounced the higher the insertion loss of both the cable connectors and the access couplers. It is, however, relatively insensitive to the insertion loss of the star coupler. Systems which employ single fibers as a communication channel do not suffer from a packing fracture problem, and both the cable connectors and the access couplers can, in principle, be fabricated with extremely low insertion losses. The serial distribution format for a single strand fiber system can be assembled to serve a modest number of remote terminals without consuming an unreasonable portion of the available power budget from normally used sending and receiving equipment.
An example of a single strand coupler is found in U.S. Pat. No. 4,054,366, to Brnoski, et al. U.S. Pat. No. 3,579,316, to Dyott, et al., shows the use of a capillary tube having an appropriate infractive index, for forming an optical waveguide coupler. Such a capillary tube is heated so that part of the tube falls to a diameter which is substantially equal to that of the intended waveguide. One significant problem with such couplers is that it is necessary to account for a weakening of the fiber optic system at the location of the fusion. Since the capillary tube is reduced in thickness, the point of fusion becomes particularly weak and may easily fracture. Furthermore, it is necessary to provide strain relief for the optical fibers in order to prevent the coupler from fracturing the optical fibers when the coupler is placed into service.
Accordingly, it is desired to provide a process to control the manufacture of a fiber optic component which transfers or splits optical energy traveling through one fiber optic waveguide into two waveguides.